_stats

_stats calculates some numerical statistics on your emitted values, which must be numbers.

The reduce output is an object that looks like this:

{"sum":2,"count":2,"min":1,"max":1,"sumsqr":2}

"sum" and "count" are equivalent to the _sum and _count reductions. "min" and "max" are the minimum and maximum emitted values. "sumsqr" is the sum of the squares of the emitted values (useful for statistical calculations like standard deviation).